1. Field of Invention
The present invention is concerned with local area networks, and more particularly, with fiber optic token bus networks utilizing active and passive star couplers.
2. Description of the Prior Art
Fiber optic networks utilize various architectures or interconnect techniques for providing communication access between all nodes or users on the network. Configurations such as rings, tapped linear buses and star coupled buses are common examples of such architectures. Star coupled networks have several known advantages over other types of networks. An optical star coupler is a passive, optical mixing element of either a transmissive or reflective variety. The transmissive star coupler (which uses a fusion region for the signal mixing) is the more common of the two. Any signal entering an input port of a passive star is divided and sent to all output ports, creating an effective bus interconnection. There is, however, a practical limit to the size (i.e., number of ports) that can be constructed, an exemplary number being 64.
In order to extend the number of users that may access the network, it is possible to interconnect several passive star sub-networks with an active star (one using active, as opposed to entirely passive, components). An active star is typically used because a passive "hub" would result in multiple signal reflections of messages or signals that originate anywhere on the composite network. Thus, some "intelligence" must be provided in the central sub-network combiner (active star) to provide proper signal routing, while preventing unwanted reflections.